[libpng16] ci: Verification should pass regardless of autoconf artifacts

Allow ci_verify_configure.sh to pass regardless whether the configure
script and the associated configure control files are present.
This is necessary for the branches newer than 'libpng16', where these
artifacts are no longer auto-generated by default.

This is a cherry-pick of commit 09ec97e
from branch 'libpng18'.

ci/ci_verify_configure.sh

@@ -122,6 +122,10 @@ function ci_build {
         ci_spawn export CFLAGS="${CFLAGS:-"-O2"} -fsanitize=$CI_SANITIZERS"
         ci_spawn export LDFLAGS="${LDFLAGS}${LDFLAGS:+" "}-fsanitize=$CI_SANITIZERS"
     }
+    # Spawn "autogen.sh" if the configure script is not available.
+    [[ -x "$CI_SRC_DIR/configure" ]] || {
+        ci_spawn "$CI_SRC_DIR/autogen.sh" --maintainer
+    }
     # And... build!
     ci_spawn mkdir -p "$CI_BUILD_DIR"
     ci_spawn cd "$CI_BUILD_DIR"